To add a bit to the ESL plus OTL paradigm, Sound Lab, by using two audio step up transformers, one for bass and one for treble, and by therefore using a passive crossover between them, are somewhat unique in spoiling the natural match of ESLs to OTLs, because of that midrange impedance dip thus incurred. Scuttlebutt was and is that SL use solid state amplifiers (Parasound JC1s, it is said) to voice the speaker and so are ignoring the problem, essentially, for tube aficionados. Historically, ESLs like Quad, KLH9, Jantszen, Acoustat, original CLS by M-L were high impedance speakers well suited to OTLs. Then M-L revised the CLS (which came to be known as CLS II) such that impedance was lowered considerably, to suit the rising popularity of SS amplifiers, I guess. I traded in my beloved CLSs for the CLS II on account of the favorable publicity but without knowing what M-L had done to the impedance curve; the CLS II sounded awful with my Futterman OTLs. Sound Lab "borrowed" the two transformer idea from Acoustat, actually, in the 90s. So I don't know when Acoustat were driven via only one step-up transformer as you mention. The earliest Acoustat I recall was the Acoustat X, a giant panel that was direct driven by an on-board amplifier, which I heard once but cannot remember aurally. That was in the mid to late 70s.
richardbrand, Though no one ever told me this, it was my assumption that the Quad 57 is curved in the vertical plane, because it was designed to sit only a few inches off the floor. So they devised a stand that leans the speaker back a bit to tilt it upward, and to further enhance radiation to the height of a listener's ears, they curved the panel. I imagine that all this was to accommodate the smaller listening spaces of typical UK houses and apartments. They never dreamed of stacked pairs as part of the original design intent. I would guess that the last thing anyone thought about was how the vertical curve would focus the rear radiation.
Also, now I think of it, you are probably correct. The 1963 speaker was called "ESL63". It falls trippingly from the tongue. The ESL63 is the only Quad speaker I ever owned. I didn't love it as much as some others. I just don't recall the 1957 speaker being referred to as ESL57, but if you say so....
